Resumen de Nutritive quality of Chloris gayana leaf blades in vegetative stage. Effect of leaf size reduction and ageing
Roxana Edith Avila, Oscar Norberto di Marco, Mónica Agnusdei
The indigestible fiber that accumulates with increasing leaf biomass affects dry matter digestibility (DMD) and voluntary intake of forages. This study sought to evaluate accumulation of neutral detergent fiber (NDF), indigestible NDF (iNDF), and apparent DMD in vitro of leaf blade of two sizes (long, L and short, S) of Chloris gayana K. cv. Finecut, with the aim of determining whether the accumulation of NDF or iNDF is dependent on leaf size. Two phases of a greenhouse experiment were conducted in 2008. In the first, three seeded microswards in 1 x 3.5 x 3 m, divided in two, were submitted to two different defoliation intensities for 33 d (24/01 to 26/02) to obtain plants of the two leaf blade sizes. In phase two, herbage grown for 40 d (27/02 to 7/04) at a mean ambient temperature of 24 ± 3ºC was used to harvest about 200 shoots at different life stages at intervals following 58 ± 22 degree-days of growth (DDG). Base temperature for calculating leaf age (expressed as (DDG) was 12°C. Harvested material was frozen immediately (liquid N2) and stored for subsequent analysis. After thawing, blades were grouped by three successive leaf generations (G1, G2, and G3) and six age categories: two of early growth, one recently expanded with visible ligulae, and three at prescenesence adult maturity stages. Blade and sheath length were determined in all samples, whereas additional criteria, including NDF and 24-h in vitro incubation to estimate residual iNDF(iNDFr), were used with samples of the last four age categories. The iNDF valve was calculated as iNDFr/NDF and DMD as the difference between incubated DM and iNDFr, minus a metabolic factor of 119 mg/kg. The experimental design was split plots in completely randomized blocks (n = 3); principal plots were leaf size (n = 2) and subplots were leaf generation (n = 3). Relationships betweem NDF, iNDF, and DMD and age (DDG) specific leaf area (SLA) and leaf length were analyzed by linear regression. Defoliation decreased the maximum leaf length by 32% (885 ± 17 to 606 ± 15 mm) and SLA by 20% (102 ± 7 to 83 ± 8 cm2/g). Reduced leaf length was accompanied by reductions (P < 0.0001) of 3% in NDF content and 13% in iNDF and an increase of 10% in DMD. Variability in iNDF was more closely correlated with leaf length (R2 = 0.62) than with SLA (R2 = 0.53). Both variables explained a similar proportion of DMD variability (R2 = 0.45) and were not associated with NDF content. Ageing of leaf blades exerted different effects in each treatment. In S it was unrelated to NDF, iNDF, and DMD, but in L it was associated with 8% increased iNDF and 6% decreased DMB, absent an effect on NDF content. Conclusions were: (1) a reduction in leaf length increased the nutritive value of adult leaf blades of Chloris gayana; (2) the improved quality of shorter leaf blades was unaffected by ageing; (3) variability in quality was more clearly associated with leaf length than with ageing.
